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*e-mail: maryanavieira@hotmail.com
Slurry Sampling for Arsenic Determination in Sediments by Hydride Generation Atomic
Absorption Spectrometry
Daiane P. Torres, Mariana A. Vieira,* Anderson S. Ribeiro and Adilson J. Curtius
Departamento de Química, Universidade Federal de Santa Catarina, 88040-900, Florianópolis-SC, Brazil
Um método simples e robusto para a determinação de arsênio em sedimentos por espectrometria de absorção atômica com geração de hidreto (HG-AAS) usando a amostragem em suspensão é proposto. Água régia (5% v/v) e ácido fluorídrico (1% v/v) foram misturados a uma alíquota do sedimento com tamanho de partícula 50 µm e colocados no banho ultra-sônico por 30 min. A suspensão foi deixada em repouso por 48 h para uma extração eficaz do analito para a fase aquosa. O volume final da suspensão foi ajustado com HCl 1 mol L-1. Uma
solução de NaBH4 estabilizada com NaOH foi usada como agente redutor. A arsina foi transportada ao tubo T de quartzo aquecido a 900 °C para detecção. Cinco materiais de referência certificados de sedimento foram analisados e as concentrações obtidas foram concordantes com os valores certificados, de acordo com o teste-t para um nível da confiança de 95%, com RSD (desvio padrão relativo) menor que 13%. O limite de detecção (LOD) na amostra foi de 0,6 µg g-1. O procedimento proposto para a determinação de arsênio em
sedimentos é preciso, exato e adequado para a análise de rotina de amostras ambientais.
A simple and robust method for the determination of arsenic in sediments by hydride generation atomic absorption spectrometry (HG-AAS) using slurry sampling is proposed. Aqua regia (5% v/v) and hydrofluoric acid (1% v/v) were mixed with an aliquot of the sediment with particle size of 50 µm in an ultrasonic bath for 30 min. The slurry was allowed to stand for 48 h for an effective extraction of the analyte to the aqueous phase. The final volume of the slurry was made up with HCl 1 mol L-1. A NaBH
4 solution stabilized with NaOH was used as reducing
agent. The arsine was transported to the quartz T-tube heated at 900 °C for detection. Five certified reference materials of sediment were analyzed and the obtained concentrations were in good agreement with the certified values, according to the t-test for a 95% confidence level with RSD (relative standard deviation) lower than 13%. The limit of detection (LOD) in the sample was 0.6 µg g-1. The proposed procedure for arsenic determination in sediment is precise,
accurate and adequate for the environmental samples routine analyses.
Keywords. arsenic, sediment, slurry sampling, hydride generation, atomic absorption spectrometry
Introduction
The presence of arsenic (As) in the environment and in food comes from natural sources or anthrogenic effects, such as in agriculture and in wood conservation. Especially important are the thermoelectric power plants emissions, in which arsenic is a by-product launched to the atmosphere and to the ground. The sediments act as a deposit and also as a source of pollution for the aquatic environment, because trace elements may be transformed into soluble and potentially more toxic species through
degradation or reaction. It is very important to know the concentration of potentially hazardous trace elements,
such as arsenic, due to its environmental problems.1-4
Several analytical techniques and procedures for quantification of trace elements, usually in low concentrations in environmental samples, are applicable to arsenic determination. The hydride generation (HG) is a highly efficient sample introduction technique for atomic absorption spectrometry (AAS). Usually, HG technique is based on the
reaction of NaBH4 with the acidified sample and the
separation of the analyte, as an hydride from the matrix before measurement. This is one of its main advantage, as it
significantly reduces the possibility of interferences.5,6 An
reported by Guo et al.7,8 who described the generation of
volatile forms of a number of elements by photoreduction. This process can be detected by inductively coupled plasma mass spectrometry (ICP-MS) and uses organic acids with low molecular weight to promote the production of radicals that generate volatile species of the analyte. However, this procedure should not be easily applicable to slurries that are prepared with mineral acids.
The sample preparation procedures for arsenic determination are very critical because of its high volatility. Usually, sediments are brought into solution using alkaline fusion or acid digestion before trace element determination, but these procedures are time consuming and require relatively high amounts of reagents. However, this can introduce contaminants to the sample solution and loss of volatile elements, like arsenic. Slurry sampling seems to be an attractive alternative for sample preparation. Partial extraction of the analyte into the slurry water phase is obtained, thereby improving the precision. Particle size, acid nature and concentration and calibration are important factors to be considered when slurry sampling is employed. In addition, sonication helps homogenization and extraction of the analyte and other components from the particles into
the slurry water phase.9 Alves et al.10,11 carried out the
determination of Cu, Fe, Mn and Zn in Antarctic krilland Cu, Pb and Zn in sediment, using slurry sampling in combination with flame atomic absorption spectrometry.
Borges et al.12 used slurry sampling for the determination
of Cd in sediment and sewage sludge by electrothermal atomic absorption spectrometry using Ir as a permanent
modifier. Ribeiro et al.13 reported that slurry sampling can
also be successfully applied to determine Hg in different environmental samples (sediments, sewage sludge, coal and coal fly ash) by cold vapor atomic absorption spectrometry using a batch system and a quartz T-tube.
In 1988, Haswell et al.14 investigated a procedure using
slurry sampling to determine As in soils, sewage sludge and fly ash samples by HG-AAS. The samples were prepared using HCl at room temperature. However, the developed procedure did not show satisfactory results for quantitative analyses, as the recoveries of the expected concentration values were too low: between 22.1% and 62.1%. Mierzwa
and Dobrowolski15 also developed procedures for arsenic
determination in sediment sample using a slurry prepared with nitric acid, followed by application of ultrasonic energy and microwave to extract the analyte to the liquid phase. However, this procedure needed the addition of L-cysteine to reduce As(V) to As(III) before the determination in the supernatant. Due to the several steps mentioned, Gürleyük et al.16 developed a procedure using flow injection to speed up and simplify the procedure.
The use the graphite furnace for in situ retention and determination of the hydride-forming element,
denominated HG-GF AAS is more recent.17 The
possibility of pre-concentrating the hydride generated from a large sample volume and, particularly, from slurry or from multiple sample aliquots in the graphite tube, followed by a single release of the trapped analyte by heating the furnace, is an interesting approach.
Matusiewicz and Mroczkowska18 proposed a slurry
sampling hydride generation method for As(III) and total inorganic arsenic determinations, using slurry sampling with ultrasonication and ozonation. The species of arsenic were trapped on a pre-heated graphite
furnace at 300 °C, treated with 150 µg of iridium as
chemical permanent modifier and determined by graphite furnace atomic absorption spectrometry.
Recently, Vieira et al.19 proposed a method to
determine As in sediments, coal and fly ash slurries after ultrasonic treatment by HG-GF AAS and trapping in an iridium-treated graphite tube. After grinding the samples
to a particle size of ≤50 µm, the sample powder was mixed
with aqua regia and hydrofluoric acid in an ultrasonic bath for 30 min. The LODs for arsenic in the samples
were 0.5 and 0.7 µg g-1 for the coal and sediment samples,
respectively.
The purpose of this work was the development of a simple and robust method for arsenic determination in sediment using slurry sampling and detection by HG-AAS with a quartz T-tube heated in an air-acetylene flame. The conditions used in the slurry preparation and in the hydride generation were optimized.
Experimental
Instrumentation
cell with a path-length of 165 mm and a diameter of 12 mm was heated to approximately 900 ºC in an air-acetylene
flame, with gas flow rates of 7.0 L min-1 (air) and 1.5 L
min-1 (acetylene). Integrated absorbance (peak area) was
used for signal evaluation. Samples slurries were sonicated using a Model T7 ultrasonic bath (Thorton, SP, Brazil) at room temperature.
Reagents and references materials
All chemicals used were of analytical reagent grade and the solutions were prepared using high-purity water
with a resistivity of 18.2 MΩ cm, obtained from a
Milli-Q Plus water purification system (Millipore, Bedford, MA). The nitric acid (Carlo Erba, Milan, Italy) and the hydrochloric acid (Merck, Darmstadt, Germany) were doubly distilled in a quartz sub-boiling apparatus (Kürner Analysentechnik, Rosenheim, Germany). Hydrofluoric acid (Merck) was purified by distillation in a PTFE sub-boiling still (Kürner Analysentechnik). The reductant, a 3.0 % m/v sodium tetrahydroborate solution, was prepared
by dissolving NaBH4 (Merck) in 1.0 % m/v NaOH (Merck)
and stored in a polyethylene flask under refrigeration. The calibration solutions with concentrations of 5.0 to 50.0
µg L–1 of arsenic were obtained by appropriate dilution of
a stock standard solution of arsenic, which was prepared
from high-purity As2O3 (Spex, Metuchen, NJ) for As(III).
The following certified reference materials were used: PACS-2 and MESS-2 marine sediments (National Research Council of Canada, Ottawa, ON, Canada), SRM 2704 Buffalo River sediment and SRM 1646a estuarine sediment (National Institute of Standards & Technology, NIST, Gaithersburg, MD, USA), and RS-3 river sediment,
from a round robin test.20
Plastic and glass containers were washed with tap water and diluted Extran solution (Merck), kept in contact with
10.0% v/v HNO3 for at least 48 h and rinsed three times
with deionized water prior to the use.
Slurry preparation
Based on the work of Vieira et al.,19 the following
procedure was tested. All reference materials were ground
in an agate mortar and passed through a 50 µm polyester
sieve (SEFAR®, Switzerland). Approximately, an amount of
50 mg of the ground and sieved sample was mixed with 2.5 mL of concentrated aqua regia and 0.5 mL of concentrated hydrofluoric acid in a 50 mL polyethylene flask, except for SRM 1646a, which was made up to 15 mL and placed in an ultrasonic bath for 30 min. The slurry was allowed to stand at room temperature for 48 h under occasional shaking. Then
the final volume was completed with 1.0 mol L-1 HCl, and
the slurry was manually agitated for 30 s, just before the determination, to assure homogeneity. In this way, the final concentration of the acids in the slurry was 5.0 % v/v aqua regia, 1.0% v/v hydrofluoric acid and 8.3% v/v hydrochloric acid. These concentrations of acid were optimized
previously19 and the results confirmed that these amounts
are sufficient to react with the sample and promote partial extraction of the analyte to the liquid phase.
Analytical procedure
A volume of 1 mL slurry aliquot was placed into the reaction flask of the MHS-15 system and the reducing
solution (NaBH4) was added for 10 s. The arsine vapor
generated was transported to the heated quartz T-tube cell adjusted to the AAS spectrometer. Calibration was carried out using aqueous standard solutions with As(III) in the
concentration range of 5.0-50.0 µg L-1, containing the same
acid concentrations as in the slurries (5.0 % v/v aqua regia, 1.0 % v/v HF and 8.3 % v/v HCl).
Results and Discussion
Hydride generation system
The batch system used for the hydride generation is of commercial origin, being all parameters optimized by the manufacturer for the analysis of aqueous solutions in diluted acid media. Some conditions, such as sample volume and concentration of the reducing solution were re-optimized for the slurries, using three certified reference materials of sediment: PACS-2, SRM 2704 and SRM 1646a.
Firstly, the sample volume in the reaction vessel was evaluated. The volume of slurry, of course, can significantly affect the sensitivity of the method using hydride generation. The effect of the volume of slurry was studied in the range from 1 to 5 mL using slurries of different sediments. As it is shown in Figure 1, large volumes of slurry produce integrated absorbance signal intensities above 1. A volume of 1 mL of slurry was adopted for the analyses.
The effect of the NaBH4 concentration on the As
absorbance signal was studied in the range from from 0.5 to 6.0% m/v using slurries of sediment, as it can be observed in Figure 2. The integrated absorbance signal for As in the
slurries increases with the increase of NaBH4 concentration
up to 1.5 % m/v, leveling off for higher concentration of the reducing solution, hence, a concentrations of 3 % m/v
of NaBH4, stabilized with 1% m/v of NaOH, was adopted
Effect of the slurry preparation time
A mixture of aqua regia and hydrofluoric acid was used in the slurry preparation, with the intention of improving the extraction of the analyte to the aqueous phase, in order to
reach a better precision.19 Certainly, the reactions with NaBH
4
during the hydride generation, after the slurry preparation, contributes for the extraction. The use of hydrofluoric acid is justified by the usually high silica content in sediments.
The slurries were allowed to stand at room temperature for 0, 15, 24 and 48 h. The As recoveries related to the certified values, as shown in Figure 3, were not satisfactory for all samples when time was 0, 15 or 24 h, indicating that part of the As was not released from the solid particles, even under the reactions of the vapor generation. Using a standing time of 48 h, all samples shown about 100%
recovery. Thus, the sediment slurries were prepared using 5.0% v/v aqua regia and 1.0% v/v HF and a standing time of 48 h to promote an effective extraction of the analyte to
the liquid phase. According to previous work,19 the As
recovery in the acid liquid phase of the slurries, obtained after centrifugation, were in the range 59-96% in five certified sediments, demonstrating that quantitative analysis should be carried out in the slurry and not in its liquid phase.
According to Petrick and Krivan,21 a concentration of
up to 1% v/v HF present in hydrochloric acid sample solution does not affect the hydrogenation of As(III). Thus, the HF concentration must be controlled to avoid the
formation of fluor-complexed ions, such as [AsF5OH]–,
which do not react with NaBH4. These complexes can
also be hydrolyzed by prolonged treatment with HCl.21
Effect of the oxidation state
It is known that, using the batch system and a pH ≤ 1,
the arsine is formed more slowly from As(V) than from
As(III).21 This results in peak heighs that are 25 to 30%
lower for As(V), but peak areas are approximately the
same for both species.21 In this way, pre-reduction of As(V)
can be omitted for simplicity when these species are present after sample treatment, if peak area is used for quantification, as it was the case in this work.
Figures of merit and analytical application
Calibration curve, using As(III) standard solutions in the
concentration range 5-50.0 µg L–1, was obtained, using the
optimized conditions and acid medium employed in the preparation of the slurry. The obtained correlation coefficient
1 2 3 4 5
0.0 0.5 1.0 1.5 2.0 2.5 3.0
In
te
g
rat
ed
absor
ba
nce/
s
Volume of slurry/mL
Figure 1. Effect of slurry volume on the integrated absorbance signal by HG AAS for As in the slurries of different certified sediments. (--) PACS-2 25.8 µg L-1, (-c-) SRM 2704 23.9 µg L-1 and (-U-) SRM 1646a 21.0 µg L-1 mean
± standard derivation, n = 3.
Figure 2. Effect of NaBH4concentration on the integrated absorbance signal for As in the slurries of different certified sediments. (--) PACS-2 25.8 µg L-1, (-c-) SRM 2704 23.6 µg L-1 and (-U-) SRM 1646a 20.6 µg L-1.
0 1 2 3 4 5 6
0.0 0.2 0.4 0.6 0.8 1.0 1.2
In
te
gr
at
ed
absor
bance/
s
NaBH%/ (m/v)
Figure 3. Effect of time preparation of the slurries on the recoveries of the certified values for As in the slurries of different certified sediments mean ± standard derivation, n = 3.
0 20 40 60 80 100
R
ec
ov
er
y
of
conc
ent
rat
ion
val
ue
(%
)
time / h
SRM 1646a SRM 2704
PACS-2 RS-3
(r2) was 0.9997 and the slope was 0.0612 s L µg-1. The limit
of detection (LOD), calculated as three times the standard deviation of ten measurements of the blank divided by the
slope of the calibration curve, was 0.6 µg g-1. This detection
capacity was adequate for the analysis of the certified sediments, using the proposed slurry preparation procedure. The accuracy of the procedure was verified by determining As in five certified sediment samples. As shown in Table 1, the found concentrations are in agreement with the certified values, according to the t-test for a confidence level of 95%. The relative standard deviations (RSD), which are also shown in Table 1, were lower than 13% for four of the five samples, indicating an adequate precision for slurry sampling in this concentration range. Certainly, real samples of sediment could also be analyzed, since the concentrations found in
another work19 for samples collected in three locations around
the Santa Catarina Island, Brazil were at least ten times above the LOD found in this work.
The good results obtained is an indication that almost 100% of the analyte in the sample reacts with the reducing agent, indicating an effective extraction of the analyte to the aqueous phase aided by the acid medium of the slurry and also by the reactions with the reducing agent. Kinetic effects from As species with different oxidation states were accounted for, as integrated signals were used.
Conclusions
Slurry sampling combined with the classical arsine generation atomic absorption spectrometry showed to be a simple method for the quantitative analysis of sediments with adequate precision and accuracy. The standing time is a very important parameter to obtain accurate results. Slurry sampling avoids analyte loss and is very simple, not requiring special instrumentation for sample preparation. Certainly, the method is applicable to other environmental samples.
Acknowledgments
The authors are thankful to the Conselho Nacional de Pesquisas e Desenvolvimento Tecnológico (CNPq)
and to the Financiadora de Estudos e Projetos (FINEP) for financial support. The authors have research scholarships from CNPq.
References
1. Hudson-Edwards, K. A.; Houghton, S. L.; Osborn, A.; Trends
Anal. Chem.2004, 23, 745.
2. Azevedo, F. A.; Chasin, A. A. M.; Metais: Gerenciamento da
Toxicidade, 1st ed., Atheneu: São Paulo, 2003.
3. Pataca, L. C. M.; Botoleto, G. G.; Bueno, M. I. M. S.; Quim.
Nova 2005, 28, 579.
4. Barra, C. M.; Santelli, R. E.; Abrão, J. J.; de la Guardia, M;
Quim. Nova 2000, 23, 58.
5. Dědina, J.; Tsalev, D. L.; Hydride Generation Atomic
Absorption Spectrometry, John Wiley: Chichester, 1995.
6. Moretto, A. L.; Cadore, S.; Mikrochim. Acta2004, 146, 239. 7. Guo, X.; Sturgeon, R. E.; Mester, Z.; Gardner, G. J.; Anal. Chem.
2004,76, 2401.
8. Guo, X.; Sturgeon, R. E.; Mester, Z.; Gardner, G. J.; J. Anal.
At. Spectrom. 2005, 20, 702.
9. Bendicho, C.; Loos-Vollebregt, M. T. C. de; J. Anal. At.
Spectrom. 1991,6, 353.
10. Alves, F. L.; Smichowski, P.; Farías, S.; Marrero, J.; Arruda, M. A. Z.; J. Braz. Chem. Soc.2000,11, 365.
11. Alves, F. L.; Cadore, S.; Jardim, W. F.; Arruda, M. A. Z.; J.
Braz. Chem. Soc.2001,12, 799.
12. Borges, D. L. G.; Dias L. F.; Da Veiga, M. A. M. S.; Curtius, A.
J.; J. Braz. Chem. Soc.2003,14, 291.
13. Ribeiro, A. S.; Vieira, M. A.; Curtius A. J.; J. Braz. Chem. Soc. 2004,15, 825.
14. Haswell, S. J.; Mendham, J.; Blutler, M. J.; Smith, D. C.; J.
Anal. At. Spectrom. 1988, 3, 731.
15. Mierzwa, J.; Dobrowolski, R.; Spectrochim. Acta Part B 1998, 53, 117.
16. Gürleyük, H.; Tyson, J. F.; Uden, P. C.; Spectrochim. Acta Part B 2000, 55, 935.
17. Matusiewicz, H.; Sturgeon, R.E.; Spectrochim. Acta Part B 1996, 51, 377.
18. Matusiewicz, H.; Mroczkowska, M.; J. Anal. At. Spectrom. 2003, 18, 751.
19. Vieira, M. A.; Welz, B.; Curtius, A. J.; Spectrochim. Acta Part B 2002,57, 2057.
20. Heininger, P.; Pelzer, J.; Henrion, R.; Henrion, G.; Fresenius’
J. Anal. Chem.1998,360, 344.
21. Petrick, K.; Krivan, V.; Anal. Chem.1987, 59, 2476. 22. Welz, B.; Sperling, M.; Atomic Absorption Spectrometry, 3rd
ed., Wiley: Weinheim, 1999, 437.
Received: October 17, 2006
Web Release Date: June 19, 2007 Table 1. Analytical results for arsenic determination in certified reference
sediment using slurry sampling HG-AAS. Obtained values (average ± standard deviation) in µg g-1 and n = 3
Samples Certified Determined RSD (%)
RS-3 16.4 ± 0.50 16.1 ± 0.7 4.3
PACS-2 26.2 ± 1.50 25.6 ± 3.4 13.3
SRM 2704 23.4 ± 0.80 23.3 ± 2.7 11.6
SRM 1646a 6.23 ± 0.21 6.24 ± 1.17 5.6
